The present invention relates in general to the technological field of superconducting materials and components and, more specifically, relates to a method of manufacturing superconducting tapes or wires based on MgB2.
Superconductivity is a phenomenon which has important applicational aspects, above all with regard to the production of high static magnetic fields. Amongst the large-scale industrial applications of superconducting materials, when they are processed in the form of superconducting wires, are magnets for medical magnetic resonance MRI (in which the static magnetic field generated by the superconductor reaches a value of 1-3 Tesla) and for NMR spectrometry (in which the generated static magnetic field reaches the value of 20 Tesla).
The superconducting materials which are most commonly used in industrial applications are two binary compounds based on niobium, that is NbTi and Nb3Sn. They have superconductive transition temperatures below 20K and therefore offer properties adequate to justify their advantageous use only at working temperatures of between 1.5 K and 4.2 K, which can be reached by suitable cooling in a liquid helium bath.
At the moment, the limitations of the compounds NbTi and Nb3Sn are a low critical magnetic field, which therefore limits its applicability in magnetic fields of up to about 7-9 Tesla, and great fragility, high cost and complexity of manufacture, respectively.
The recent discovery of the superconductivity phenomenon in the simple binary compound MgB2 (magnesium diboride) which has a transition temperature of about 40K (the highest ever detected for a binary compound) opens up new prospects for the application of this material in competition with the compounds which have been known for some time and optimized in the form of superconducting cables in the course of the last few decades.
This discovery is mentioned by J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani and J. Akimitsu in an article entitled “Superconductivity at 39K in magnesium diboride” which appeared in the journal Nature, volume 410, pages 63-64, published on 1 Mar. 2001.
A method of manufacturing tape conductors based on MgB2 with a metal covering by the so-called-powder-in-tube technique is known in the literature from the article by the inventors in collaboration with C. Ferdeghini, S. Roncallo, V. Braccini, and A. Siri, entitled “Large transport critical currents in unsintered MgB2 superconducting tapes”, published in Applied Physics Letters, vol. 79, No. 2, pages 230-232, of 9 Jul. 2001.
According to the teachings of the article, already-reacted high-purity powders of the compound MgB2 are poured into a metal tube and compacted therein. The tube is then processed cold by drawing and rolling to form a small-diameter superconducting wire whilst maintaining a geometrical shape of circular cross-section and, finally, to produce a tape conductor.
The conductor thus formed is constituted by an outer metal covering and an inner core based on magnesium and boron which has substantially a lattice structure formed by grains of the compound MgB2.
The conductors thus produced have a critical current density of about 105 A/cm2 at a temperature of 4.2 K.
Cold mechanical processing promotes the compaction of the compound MgB2 and its agglomeration, producing structural defects in the crystalline lattice of the MgB2 grains which improve its superconducting properties.
The article “Transport current in MgB2 based superconducting strand at 4.2 K and self-field” by M. D. Sumption, X. Peng, E. Lee, M. Tomsic and E. W. Collings published in the 2001 Preprint collection, cond-mat/0103179, describes a similar process in which a step of sintering by heating to 900° C. with a duration of 1 hour or more follows the rolling operation in order to increase the connection between the MgB2 grains making up the core of the conductor, cancelling out the structural defects produced by the mechanical processing.